Heart disease is among the leading causes of disability and mortality in United States and worldwide. After ischemic injury, human hearts cannot regenerate and heal by scarring, which gradually leads to cardiac dysfunction. Replacing the damaged myocardium with new cardiomyocytes and neovascularization may restore cardiac function. By contrast to mammals, zebrafish hearts have remarkable regenerative capacity, completely regenerating lost tissue after surgical resection. Very little is known about the molecular and cellular mechanisms of heart regeneration in zebrafish. During this process, embryonic epicardial and pericyte/mesenchymal markers are upregulated, suggesting that the epicardium is activated and undergoes an epithelial-to-mesenchymal transition (EMT) to form new blood vessels. We found platelet-derived growth factors (PDGFs) are important for heart regeneration. Blocking PDGF signaling caused impaired new blood vessel formation and decreased DNA synthesis in cardiomyocytes during zebrafish heart regeneration. We hypothesize that PDGF signaling is required for epicardial cell proliferation, epicardial EMT, and cardiomyocyte proliferation. We propose to characterize the functions of PDGFR2 signaling in epicardial cell proliferation, EMT, and new blood vessel formation in vivo (Aim 1) and in vitro (Aim 2). We further propose to determine the function of PDGFR1 signaling in zebrafish cardiomyocytes and cardiac progenitors (Aim 3). Our unique zebrafish heart regeneration model and parallel in vitro and in vivo approaches will allow us to determine the molecular and cellular mechanisms of zebrafish heart regeneration. Our long-term goal is the development of novel therapeutic approaches for heart diseases in the future.